Full-scale electric aircraft requires approximately 50 MW of electric power to be delivered to electric motor propulsors that are strategically distributed throughout the aircraft for optimal system performance. The complete power system including a turbo-generator, distribution, protection, converters, and motors all generate heat that must be dissipated. Studies suggest that over half the mass associated with a full electric power propulsion system is attributed to the thermal management system and as such is likely a key limiting factor to achieving economical flight. Moreover, as the operating temperature is reduced, the power and motor components become flight-weight and more efficient but often at the expense of increased thermal management system mass. In particular, the best system performance is predicted to occur at cryogenic temperatures. Ideally the aircraft thermal management system can lift 500 kW or more of heat from 50K to 300K with a mass of 3 kg/kw or 1500 kg overall.
Today's aircraft generators are cooled convectively with jet fuel that is readily available at ambient temperatures. This is safe because the aircraft bus voltage is below the Paschen curve at 270V. Even if the insulation fails, a spark is not likely to form and ignite nearby jet fuel. Future aircraft, however, requires a bus voltage of over 4500V to keep the overall system mass and efficiency optimized for flight. At these high voltages a spark could ignite standard jet fuel as well as the alternative cryogenic fuels such as liquid methane/hydrogen.
A second approach is to provide an inert cooling fluid such as liquid nitrogen and utilize a heat pump such as reverse Brayton to refrigerate the fluid. This, however, is difficult to achieve flight-weight systems with that approach due to system complexity, which includes coolant pumps, vacuum jacketed plumbing, size-able heat exchangers and recuperation mass. Further, such a system will deplete power from the turbo-generators to operate the turbo-alternators. For cryogenic systems it is not likely possible to directly shaft connect the warm turbo-generator to the cryogenic turbo-alternator or other combinations in which an ambient component would be connected to a cryogenic component. Other approaches such as convective air cooling are inadequate for the amount of heat lift required in full-scale electric aircraft.